The present invention relates to a process for the preparation of an aromatic carboxylic acid. More particularly, the present invention relates to a process for the preparation of an aromatic carboxylic acid of the general formula
Rxe2x80x94COOH 
wherein R comprises an aryl group having 1 to 3 benzene rings, or a substituted aryl group. Still more particularly, it relates to the preparation of aromatic carboxylic acids by oxidation of alkyl aromatic compounds having a general formula
R1xe2x80x94R2 
wherein R1=alkyl or substituted alkyl having 1 to 3 carbon atoms, and R2=aryl having 1 to 3 benzene rings, using solid catalysts containing organometallic cluster complexes of cobalt and manganese.
Aromatic carboxylic acids such as benzoic acid, phthalic acid, terephthalic acid, trimethyl benzoic acids, naphthalene dicarboxylic acids and the like are used widely as intermediates in the chemical industry. They are usually prepared from the corresponding alkyl aromatic compounds by oxidation with air in the presence of liquid phase, homogeneous catalysts like cobalt acetate, manganese acetate etc. Terephthalic acid, for example, is prepared, as described in U.S. Pat. No. 2,833,816 issued to Mid Century Coloration in 1958, by the oxidation of para-xylene by air in acetic acid solvent, at around 200xc2x0 C. and 200 psig pressure, in the presence of homogeneous, liquid phase catalysts comprising of cobalt, manganese and bromine. Various modifications and improvements of this process are utilised for the manufacture of terephthalic and many other aromatic carboxylic acids. These processes are described in U.S. Pat. Nos. 5,693,856; 3,562,318A; 5,760,288; 6,160,159; 4,329,493; 4,593,122; 4,827,025; 4,835,307; 5,087,741; 5,112,992; and EP 0 754,673 A. Comprehensive reviews of the oxidation of alkyl aromatic compounds to aromatic carboxylic acids are available by Suresh et al in Industrial Engineering Chemistry Research, volume 39, pages 3958-3997, year 2000 and W. Partenheimer in Catalysis Today, volume 23, pages 69-158, year 1995. Phthalic acid is manufactured by the aerial oxidation of ortho-xylene in the vapor phase over vanadia-based catalysts. In the oxidation of para-xylene to terephthalic acid, for example, the process begins with hydrogen atom extraction from a methyl group by the bromine atom. The resultant benzyl radical adds to O2 and proceeds through the hydroperoxide to para-tolyl alcohol, para-tolyl aldehyde and para-toluic acid. Hydrogen atom abstraction from the methyl group of toluic acid generates a secondary benzyl radical, which follows the same pathway to yield, eventually, terephthalic acid.
There are many improvements that are desirable in the presently used homogeneous, liquid phase processes for the manufacture of aromatic carboxylic acids; (1) Replacement of the homogeneous by solid heterogeneous catalysts, (2) replacement of the corrosive bromine promoters which require the use of expensive titanium steel, by non-corrosive compounds, (3) elimination or reduction of the significant acetic acid oxidation to CO and carbon dioxide (5-10 wt. % of the carboxylic acid); this can, perhaps, be achieved by the use of more efficient radical promoters which allow oxidizer temperatures to be lowered without reducing reaction rates, and (4) lowering of the concentration, in the reaction product, of intermediates which are difficult to be removed from the final, aromatic carboxylic acid product: 4-carboxy benzaldehyde is a typical example of such an intermediate which necessitates elaborate hydrogenation and recrystallisation procedures in the manufacture of purified terephthalic acid required for the polyester industry.
Jacob et al in the journal Applied Catalysis A: General, volume 182, year 1999, pages 91-96 described the aerial oxidation of para-xylene over zeolite-encapsulated salen, saltin and salcyhexen complexes of cobalt or manganese in the absence of added halogen promoters and using tertiary butyl hydroperoxide, instead of bromide ions, as the initiator at low temperatures. Significant conversion levels of para-xylene (upto 50-60%) were attained. However, the main product was para toluic acid. The yields of terephthalic acid were negligible. The feasibility of using a solid, non-Br-containing catalyst in the absence of any solvent including acetic acid for the para-xylene oxidation to toluic acid, which is the first stage in the oxidation of para-xylene to terephthalic acid, was claimed to be established.
In prior art processes for the manufacture of aromatic carboxylic acid from alkyl aromatic compounds, the alkyl aromatic compound is dissolved in acetic acid along with the homogeneous catalysts, usually the acetates of cobalt and manganese, and oxidized by an oxygen containing gas, usually air or oxygen, in the presence of a promoter like NaBr or HBr at temperatures around 200xc2x0 C. and pressures of about 200 psig. Other metal acetates have also been used. U.S. Pat. No. 4,786,753, for example teaches the use of nickel, manganese and zirconium acetates in place of the acetates of cobalt and manganese. The commercial processes have been optimized to the point where typical crude aromatic acid yield is around 96-98% weight. However, as mentioned herein before, there is scope for improvement in the practice of the process one of them being the replacement of the homogeneous catalysts by solid catalysts since the latter can be more easily separated from the reaction products. In the investigations leading to the present invention, it was found that when complexes of cobalt, manganese, nickel, zirconium or any of their combinations were supported or encapsulated or grafted in solid supports, the yields of the aromatic carboxylic acids were always low in accord with the findings of Jacob et al published earlier and mentioned herein above. Hence, the prior art catalysts, like the acetates of cobalt, manganese, nickel or zirconium, while active in the homogeneous oxidation of alkyl aromatic compounds are not sufficiently active in the solid state. It is a surprising discovery of the present invention that when the solid catalyst contains certain organometallic cluster complexes of cobalt and manganese wherein each molecule of the cluster complex contains both cobalt and manganese, then their activity in the oxidation of alkyl aromatic compounds to aromatic carboxylic acids is enhanced significantly. These novel solid catalysts while retaining all the advantages of the homogeneous catalysts, like high yields of the desired aromatic carboxylic acids in the range of 96 to 98% weight, are easily separable from the reaction products by simple filtration processes, thereby not only avoiding the tedious process of catalyst recovery characteristic of prior art processes, but also eliminating the presence of toxic elements, like cobalt, manganese, nickel etc., in the waste effluent from the process. Processes utilizing these novel solid catalysts are, hence, environmentally more beneficial. Representative of the organometallic cluster complexes of cobalt and manganese of the present invention are CoMn2(O)(CH3COO)6, Co2Mn(O)(CH3COO)6, CoMn2(O)(CH3COO)y(pyridine)z, Co2Mn(O)(CH3COO)y(pyridine)z, where y+z=9, etc. It was also found that the organic ligands in the above mentioned organometallic cluster complex namely the acetate and pyridine ligands, can be replaced by other suitable organic moieties. The critical active site ensemble responsible for the high yields of aromatic carboxylic acids in the oxidation of the alkyl aromatic compounds was the heterometallic cluster complex containing both cobalt and manganese. While the exact origin of this enhancement effect is not known in detail, it is believed that multimetallic clusters of transition metal ions are better able to activate dioxygen, O2, than monometallic and monomeric ions. The common prevalence of such heteronuclear multimetallic clusters in the O2 activating enzymatic oxygenase catalyst systems supports such a suggestion. Processes for the manufacture of aromatic carboxylic acids using solid catalysts with high, almost complete, conversion of the alkyl aromatic compound and high yields of the aromatic carboxylic acid are continually sought.
It is therefore one of the objects of the present invention to provide a process for production of aromatic carboxylic acids having high conversions and high yields.
Another object is to provide a process for the production of aromatic carboxylic acids using solid catalysts.
Still another object of the present invention is to provide a process for the production of terephthalic acid using solid catalysts consisting of organometallic clusters of cobalt and manganese.
In the investigations leading to the present invention, it was found that when complexes of cobalt, manganese, nickel, zirconium or any of their combinations were supported or encapsulated or grafted in solid supports, the yields of the aromatic carboxylic acids were always low in accord with the findings of Jacob et al published earlier and mentioned herein above. Hence, the prior art catalysts, like the acetates of cobalt, manganese, nickel or zirconium, while active in the homogeneous oxidation of alkyl aromatic compounds are not sufficiently active in the solid state. It is a surprising discovery of the present invention that when the solid catalyst contains certain organometallic cluster complexes of cobalt and manganese wherein each molecule of the cluster complex contains both cobalt and manganese, then their activity in the oxidation of alkyl aromatic compounds to aromatic carboxylic acids is enhanced significantly. These novel solid catalysts while retaining all the advantages of the homogeneous catalysts, like yields of the desired aromatic carboxylic acids in the range of 96 to 98% weight, could be easily separated from the reaction products by simple filtration processes, thereby not only avoiding the tedious process of catalyst recovery characteristic of prior art processes, but also eliminating the presence of toxic elements, like cobalt, manganese, nickel etc., in the waste effluent from the process. Processes utilizing these novel solid catalysts are, hence, environmentally more beneficial. Representative of the organometallic cluster complexes of cobalt and manganese of the present invention are CoMn2(O)(CH3COO)6, Co2Mn(O)(CH3COO)6, CoMn2(O)(CH3COO)y(pyridine)z, Co2Mn(O)(CH3COO)y(pyridine)z, where y+z=9, etc. It was also found that the organic ligands in the above mentioned organometallic cluster complex, namely the acetate and pyridine ligands, can be replaced by other suitable organic moieties. The critical active site ensemble responsible for the high yields of aromatic carboxylic acids in the oxidation of the all aromatic compounds was the heterometallic cluster complex containing both cobalt and manganese. While the exact origin of this enhancement effect is not known in detail, it is believed that multimetallic clusters of transition metal ions are better able to activate dioxygen, O2, than monometallic and monomeric ions. The common prevalence of such heteronuclear, multimetallic clusters in the O2 activating enzymatic oxygenase catalyst systems supports such a suggestion.
Accordingly the present invention relates to a process for the preparation of an aromatic carboxylic acid which comprises oxidizing an alkyl aromatic compound by contacting the said alkyl aromatic compound with an oxygen containing gas at a temperature in the range of 80 to 250xc2x0 C., in the presence of a solid catalyst for a period of 45 to 240 minutes at a pressure ranging between 200 to 550 psig, and recovering the desired product.
In one embodiment of the invention, the aromatic acid has the general formula
Rxe2x80x94COOH 
wherein R is an aryl group having 1 to 3 benzene rings, or a substituted aryl group.
In another embodiment of the invention, the alkyl aromatic compound has the general formula
R1xe2x80x94R2 
wherein R1=alkyl or substituted alkyl having 1 to 3 carbons, and R2=aryl having 1 to 3 benzene rings.
In another embodiment of the invention, the solid catalyst used is of the general formula
[MxMxe2x80x2xxe2x80x2(O)(RCOO)nLnxe2x80x2]Ynxe2x80x3
wherein M and Mxe2x80x2 are cobalt and manganese ions, x and xxe2x80x2 are each between 0 to 3, R is selected from the group consisting of an alkyl group containing 1 to 5 carbon atoms, an aryl group with 1 to 3 benzene rings, substituted alkyl and aryl group, n is between 3 to 6, L is selected from the group consisting of RCOO, pyridine, nitrogen containing organic bases, H2O and organic solvent and any like ligand, Y is a halide ion, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, BrO3xe2x88x92 or any like ion, nxe2x80x2 and nxe2x80x3 are each between 0 to 3.
In a further embodiment of the invention, the oxygen containing gas is selected from the group consisting of pure oxygen, air and a mixture of gases containing oxygen.
In yet another embodiment of the invention, the desired product is recovered by condensation and distillation.
In still another embodiment the solid catalyst is of general formula
[MxMxe2x80x2xxe2x80x2(O)(RCOO)nLnxe2x80x2]Ynxe2x80x3
wherein M and Mxe2x80x2 are transition metal ions and more specifically cobalt and manganese ions, x and xxe2x80x2 may vary from 0 to 3, R may be an alkyl group containing 1 to 5 carbon atoms or an aryl group with 1 to 3 benzene rings or substituted alkyl or aryl group, n may vary from 3 to 6, L may be RCOO, pyridine, nitrogen containing organic bases, H2O, organic solvent or any like ligand, Y may be a halide ion, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, BrO3xe2x88x92 or any like ion, nxe2x80x2 and nxe2x80x3 may vary from 0 to 3 and characterized by Table 1
The method of the present invention utilises a solid oxidation catalyst which has been found to be particularly effective for oxidation of aromatic aryl groups to the carboxyl groups in high yields. The catalyst system is a solid catalyst containing an organometallic complex of cobalt and manganese. Examples of such solid catalysts include micro and mesoporous material like, aluminosilicate zeolites, aluminophosphates, carbon molecular sieves, silica and the like, containing an organometallic complex wherein the chemical composition of each molecule of the organometallic complex includes at least one atom of both cobalt and manganese.
An alkyl aromatic compound is oxidised by contacting with an oxygen-containing gas at a temperature in the range of 80 to 250xc2x0 C., in the presence of a solid catalyst for a period of 45 to 240 minutes at a pressure ranging between 200 to 550 psig. The product is recovered by conventional methods such as condensation and distillation.
The aromatic acid is preferably of the general formula Rxe2x80x94COOH, where R is an aryl group having 1 to 3 benzene rings, or a substituted aryl group.
The alkyl aromatic compound may also have the general formula R1xe2x80x94R2 wherein R1=alkyl or substituted alkyl having 1 to 3 carbons, and R2=aryl having 1 to 3 benzene rings.
The catalyst is of the general formula [MxMxe2x80x2xxe2x80x2(O)(RCOO)nLnxe2x80x2]Ynxe2x80x3 where M and Mxe2x80x2 are transition metal ions preferably cobalt or manganese ions, x and xxe2x80x2 may vary from 0 to 3, R may be an alkyl group containing 1 to 5 carbon atoms or an aryl group with 1 to 3 benzene rings or substituted alkyl or aryl group, n may vary from 3 to 6, L, may be RCOO, pyridine, nitrogen containing organic bases, H2O, organic solvent or any like ligand, Y may be a halide ion, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, BrO3xe2x88x92 or any like ion, nxe2x80x2 and nxe2x80x3 may vary from 0 to 3.
Table 1 below gives the characterisation data of the solid Co/Mn cluster complexes.
Copending U.S. patent application Ser. No. 09/894,997 (NF 270/01) describes the catalyst system used in the instant invention and a process for the preparation thereof.
The oxygen containing gas may be pure oxygen, air or mixture of gases containing oxygen and is preferably air.
Oxidation of the alkyl aromatic compound to the aromatic carboxylic acid is done in the presence of an oxygen containing gas and a solid catalyst containing an organometallic cluster complex of cobalt and manganese. The separation of the solid crystals of the aromatic carboxylic acid from the reaction product and isolating from the solid crystals of aromatic carboxylic acid, an aromatic carboxylic acid having a purity greater than 99% by weight.
The solid catalyst has an organometallic cluster complex with at least one atom of cobalt and one atom of manganese in each molecule of the cluster complex and can be separated from the solid crystals of the product from the reaction mixture.
This invention is illustrated by the following examples, which are illustrative only, and should not be construed to limit the scope of the present invention.